RADIOACTIVITY


LIBRARY: Find out the following:

How does a smoke detector work?

How does a Geiger tube (also known as “Geiger-Müller” tube) work? What do the following terms mean in relation to it: “voltage plateau”, “avalanche region”?

What are the isotopes of americium? What are their half-lifes? What does each one emit as it decays?

What is the “dead time” of a radiation detector? How does one use a “split source” to measure dead time? Find an equation.


DETECTORS, STOPPING POWER:
Set up both a Geiger-Müller tube and a computer-interfaced “microRoentgen” radiation detector to measure the radioactivity of the americium source inside a smoke detector. Use the appropriate lab hardware to place either detector an appropriate distance above the source. Be sure to store the smoke detector assembly safely when you are done today.

For the Geiger tube, measure the count rate as a function of Voltage. Start at the lowest Voltage for which you get counts, and increase Voltage until you reach the “avalanche” regime. Do not exceed 1000V unless instructed to do so.

For both detectors, measure the maximum count rate you can get from the source. Plot the count rate vs the number of sheets of paper (0, 1, 2, 3, etc.) placed between the source and the detector. Can you fit the data to a simple model? What does this tell you? How about various identical sheets of other materials? Comment on your results. What can you conclude about the type[s] of radiation emitted by the americium?

DEAD TIME:
Some radiation detectors, like a Geiger tube, suffer from 'dead time'. After an event has triggered the tube, it is 'dead', or unresponsive for some time thereafter, and cannot respond to further events. There are three ways you can quantify the dead time, and you should do each for the GM tube. (1) From the maximum count rate. The dead time must be shorter than 1 divided by this count rate. Find an upper bound. (2) Using the oscilloscope. Look at the output of the GM tube “scaler” on the scope. By inspection, you can get a rough measure of the dead time. Be sure to use the storage scope to get a "time lapse" printout of the detector signal. (3) Using a split source. Look up how to calculate dead time for a split source, then take very careful data. You need to calculate the uncertainty of your measurement of dead time.

When you have finished, compare your three different measurements of dead time. Be sure to comment on whether they are consistent, given your uncertainties.

HALF-LIFE, DECAY RATE, AND AMOUNT OF RADIOACTIVE MATERIAL:
From purely practical considerations, determine which isotope(s) of americium are likely to be used in your smoke detector. Measure the activity of the americium (making sure to subtract the background level). Check this against the activity marked on the smoke detector itself and calculate the efficiency of your setup in counting radiation. Is the dead time of your detector to blame for any low efficiency? Estimate the total mass of americium in the source, (using the activity marked on the source). You may need to differentiate the expression for the number of remaining radioactive nuclei as a function of time to get this relation.

CLOUD CHAMBER:
Your instructor will set up a cloud chamber to observe ionization tracks in air caused by radiation, and will demonstrate how to apply an electric field or a magnetic field to the chamber. For the various types of radiation -- alpha, beta, gamma -- remark on the charge, range, and relative mass of the particular radation.

HALF-LIFE. Time permitting, we will analyze some muon decay data taken at St. Lawrence to calculate the half-life of muons.

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